Pour 300ml/10fl oz just-boiled water into a jug and stir in the half a stock cube until dissolved. Add the dried mushrooms and stir well. Leave to soak for 30 minutes.
To make the mash, peel the potatoes and cut into roughly 4cm/1½in chunks. Put in a large non-stick saucepan and cover with cold water. Bring to the boil, then reduce the heat slightly and simmer for 15-18 minutes, or until very tender.
Drain the potatoes in a colander and then return to the saucepan and mash with the butter, milk and plenty of salt and pepper until smooth. Put back on the hob over a very low heat until the pheasant filling is ready, stirring occasionally.
Tip the soaked mushrooms and their stock into a sieve over a bowl to drain and reserve the liquid. Transfer the mushrooms to a chopping board and very roughly chop. Have a look at the liquor and if it is very gritty, pass through muslin or a fine sieve and into a clean bowl.
Heat one tablespoon of the oil in a large non-stick frying pan or sauté pan and fry the fresh mushrooms and soaked mushrooms over a medium heat for two minutes, stirring occasionally. Tip onto a plate and return the pan to the heat.
Add one more tablespoon of oil to the pan and fry the bacon and onion for 2-3 minutes or until beginning to brown, stirring regularly. Season the pheasant well with salt and pepper and add to the pan with the onion and bacon. Fry for two minutes, turning the pheasant in the pan every now and then until lightly browned.
Working quickly, add the garlic and port to the pan and allow the liquid to bubble furiously for a few seconds. Sprinkle the flour over and stir through the pheasant and vegetables until lightly coated.
Gradually stir in the reserved mushroom and chicken stock liquid, scraping the bottom of the pan to lift the sediment and sticky juices. Add the mushrooms to the pan and stir in the parsley. Bring to a gentle simmer and cook for five minutes, or until the pheasant is just cooked through and the sauce has thickened, stirring regularly. Remove from the heat.
Preheat the grill to its hottest setting. Boil a kettle of water and half fill a 1.6 litre/3lb 8oz shallow ovenproof dish. The hot water will warm the dish. Leave for two minutes and then carefully tip the water down the sink. Spoon the pheasant mixture into the dish.
Spoon the mashed potato on top of the pheasant, working your way around the sides before heading into the middle. Put the pie on a baking tray and grill for 6-8 minutes, or until the potato is lightly browned. Drizzle the pie with the truffle oil, if using, and serve.